REGULATOR TEMPERATURE ANALYSIS

Pressure regulators are to be employed at a gas-fired power station

to reduce upstream gas pressures from a maximum of 15 MPa to

approximately 3.5 MPa. Due to the Joule-Thompson effect, the

resulting gas temperature drops could be in the region of 55 °C. The

dew-point temperature of the hydrocarbons (gas) flowing through is

-15 °C and the minimum ambient temperature of the area is -6 °C.

Thus the regulators could potentially be subjected to gas at -61 °C at

start-up. According to the valve manufacturer, temperatures as low

as -20 °C can be tolerated for some time, provided that condensation

does not occur.

POWER INDUSTRY

www.flownex.com sales@flownex.com

Pag

e1

CHALLENGE:

The client needed to know:

The temperature profile of the regulator seat during start-up operations.

The effect of proposed measures to prevent the seat from experiencing

such low temperatures.

BENEFITS:

Analysis of the regulator seat temperature over time and exposure to low

temperatures were calculated. The simulation results will allow the client to

determine what measures are needed to prevent adverse effects.

SOLUTION:

Operation of the pressure regulators during the start-up of the turbines at a

gas-fired power station was studied and simulated in Flownex®. The major

advantage of using Flownex® is that the capacitance of the pipe material and

the full Joule-Thompson effect could be simulated. The simulations established

that the regulator internals would be exposed to extremely cold gas for an

extended period of time. The model was also able to determine if the proposed

trace heating and insulation system would be sufficient to prevent the valve

internals from cooling below their minimum temperature limit of -20°C, with a

reasonable safety factor taken into account.

POWER INDUSTRY

“Anyone familiar with transient heat transfer of flow systems with

complicated geometries will tell you that such an analysis would be

beyond the capability of most engineering houses. Flownex

enabled me to obtain reliable ball-park results in a matter of a few

hours. When the potential cost of equipment failure is tens of

millions of dollars, this is an amazing result which highlights the

incredible power and flexibility of Flownex”

Hannes van der Walt

Senior Thermal & Process Engineer

www.flownex.com sales@flownex.com

Pag

e2

INTRODUCTION

Pressure regulators are to be employed at a gas-fired power station

to reduce upstream gas pressures from a maximum of 15 MPa to

approximately 3.5 MPa. Due to the Joule-Thompson effect, the

resulting gas temperature drops could be in the region of 55 °C.

The dew-point temperature of the hydrocarbons (gas) flowing

through is -15 °C and the minimum ambient temperature of the

area is -6 °C. Thus the regulators could potentially be subjected to

gas at -61 °C at start-up. According to the valve manufacturer,

temperatures as low as -20 °C can be tolerated for some time,

provided that condensation does not occur.

It was important to the client to analyse the regulator seat

temperature over time and evaluate the exposure to low

temperatures. The metal-alloy seat has an O-ring incorporated into

its design to achieve proper sealing. At very low temperatures, the

O-ring can deteriorate, become brittle and break. The seat itself

may also become brittle and be susceptible to warping and/or

breakage. The simulation results will allow them to determine what

measures may be needed to prevent these adverse effects.

JOULE-THOMPSON EFFECT:

Temperature change of a gas

or liquid when it is forced

through a valve or porous

plug whilst no heat is

exchanged with the

environment (adiabatic

expansion).

Figure 1: Cross section of a Pressure

Regulating Valve.

REGULATOR TEMPERATURE ANALYSIS

seat

www.flownex.com sales@flownex.com

Pag

e3

BACKGROUND

At normal operations, the gas is heated up to 65 °C by heat

exchangers upstream of the regulators. It then flows to the gas

turbine where it is used as fuel (See Figure 2). However in the first

minute of the start-up process, the mass flow rate is a lowly 1.5 kg/s

per gas turbine. Thus the potentially cold gas contained in the pipe

spool between the heat exchanger and the regulator will expose

the regulator seats to extremely low temperatures for an extended

period before the heated gas arrive at the regulators.

Heat Exchanger Regulator Valve Turbine

20 m DN200 80 m DN200

Figure 2: Flow path of the gas

SOLUTION

A Flownex® model in Figure 3 was developed to model the flow

from the outlet of the heat exchangers through 20 m of piping to

the outlet of the regulators. Temperatures vary along the length of

the pipe. Thus to ensure sufficient accuracy of heat transfer from

the gas to the pipe, it is subdivided into 20 increments. Gas flows

through the system at the initial start-up flow rate of 1.5 kg/s for

the first 60 seconds by opening the regulator orifice to 9 mm to

achieve this. The Reynolds number using the fluid properties from

the orifice were calculated. The convective heat transfer coefficient

at the regulator exit through use of the Dittus-Boelter equation was

then determined.

The benefits of using Flownex® for the simulation were:

Heat conduction was simulated in 2D, lateral and cross

conduction in the pipe wall.

The thermal capacitance or ability of a material to store and

release heat during transient events can be modelled to

determine the actual start up conditions and the

temperatures experienced by the regulator valve.

Piping data could be obtained from the built-in pipe

schedule tables. This ensure less time was spent on the

setting up of the model and more time was available to

evaluate and improve the design and operation of the

“Temperatures vary

along the length of the

pipe. Thus to ensure

sufficient accuracy of

heat transfer from the

gas to the pipe, it is

subdivided into 20

increments.”

“The convective heat

transfer coefficient at

the regulator exit

through use of the

Dittus-Boelter equation

was then determined.”

www.flownex.com sales@flownex.com

Pag

e4

system to protect the turbine from over pressurisation and

low temperatures.

User coding could be used to calculate the heat transfer

coefficient of the gas at the regulator exit to accurately

model the Joule-Thompson effect on the valve seat.

The temperature profiles can be displayed graphically in

Flownex® for easier interpretation of results.

Figure 3: The Flownex® model used for analysis.

A simulation run without trace heating and a simulation run

with trace heating around the piping could be saved as

snaps within one model. This feature can be used where

different materials, fluids, boundary conditions, etc are to be

compared on the same model without needing to set up

each scenario and results from scratch each time or in

different networks.

The client could determine that the minimum temperature

limit for the regulator seats would be exceeded due to the

cold gas if no trace heating was used. Preventative

measures were taken and in doing so, the client avoided a

potentially disastrous equipment failure incident.

The model initialises all material and the contained gas to -6°C and

then allows gas at 60°C to enter the upstream piping from the heat

exchanger exit. Temperatures determined from the analysis are

shown in Figure 4.

It is thus possible for the average regulator seat temperature to

drop to unacceptably low temperatures (-40°C) for some time

“The model initialises all

material and the

contained gas to -6°C

and then allows gas at

60°C to enter the

upstream piping from

the heat exchanger

exit“

Seat

www.flownex.com sales@flownex.com

Pag

e5

during start-up. The regulator inlet gas temperature will remain at -

6°C for approx. 40s before heated gas from the heat exchangers

arrive. The figure also shows that the regulator exit temperature is

predicted to be at -63°C for about 40s before it starts to rise, and it

will still be sub-zero after two minutes. These extremely low

temperatures can have very catastrophic consequences.

Figure 4: Flownex® temperature results of the gas and the valve seat.

It was suggested that to prevent the gas from reaching subzero

temperatures, trace heating be implemented on the pipe spool

between the heat exchanger and the regulators.

The same Flownex® model was used for analysis, but all materials

and the contained gas are initialised to 50°C and then gas at 60°C

is allowed to enter from the heat exchanger exit. Figure 5 shows the

predicted temperatures with trace heating installed.

None of the temperatures dropped below the regulator design

temperature, and the Joule-Thompson effect causes the gas

temperature in the regulator to drop to only about 12°C in the first

40s. The analysis shows that a trace heating system that will keep

the gas temperature above 12°C would be adequate to protect the

valve seats.

SUMMARY

Operation of the pressure regulators during the start-up of the

turbines at a gas-fired power station was studied and simulated in

Flownex®. The major advantage of using Flownex® is that the

capacitance of the pipe material and the full Joule-Thompson effect

could be simulated. The simulations established that the regulator

internals would be exposed to extremely cold gas for an extended

“the Joule-Thompson

effect causes the gas

temperature in the

regulator to drop to

only about 12°C in the

first 40s “

www.flownex.com sales@flownex.com

Pag

e6

period of time. The model was also able to determine if the

proposed trace heating and insulation system would be sufficient to

prevent the valve internals from cooling below their minimum

temperature limit of -20°C, with a reasonable safety factor taken

into account.

Figure 5: Flownex® temperature results of the gas and the valve seat

(with trace heating).

INDUSTRIAL GAS TURBINES BACKGROUND

A gas turbine, also called a combustion turbine, is a rotary engine

that extracts energy from a flow of combustion gas. Energy is

added to the gas stream in the combustor, where fuel is mixed with

air and ignited.

The construction process for gas turbines can take as little as

several weeks to a few months, compared to years for base load

power plants. Their other main advantage is the ability to be turned

on and off within minutes, supplying power during peak demand.

Since single cycle (gas turbine only) power plants are less efficient

than combined cycle plants, they are usually used as peaking power

plants, which operate anywhere from several hours per day to a few

dozen hours per year, depending on the electricity demand and

the generating capacity of the region.

Natural gas is currently one of the most widely used fuel for new

power plants, mainly due to attractive gas pricing, low emissions

and the favourable capital costs of gas turbine power plants.

“The model was able to

determine if the

proposed trace heating

and insulation system

would be sufficient to

prevent the valve

internals from cooling

below their minimum

temperature limit of -

20°C, with a reasonable

safety factor taken into

account “